Described herein are a method and an apparatus for heat treating and processing of metals, such as but not limited to steels, in a carbon-containing atmospheres. More specifically, described herein is a method and apparatus for carburizing, carbonitriding, nitrocarburizing, controlled carbon potential annealing, softening, brazing and sintering that may be conducted, for example, in a one atmosphere-pressure, batch or continuous furnace.
Conventional carbon-containing atmospheres are generated in endothermic and, sometimes, exothermic generators that are remote or external to heat-treating furnaces. The carbon-containing atmospheres are oftentimes adjusted to match processing requirements by mixing with one or more hydrocarbon gases (HC) such as methane (CH4), propane (C3H8), propylene (C3H6), acetylene (C2H2), ammonia (NH3), and/or nitrogen (N2). Since endothermic gas reformed, most frequently, with air forms hydrogen (H2), N2, and carbon monoxide (CO), with minute quantities of water vapor (H2O), and carbon dioxide (CO2), the conventional atmospheres have a potential to oxidize alloying additions present in steel, e.g. chromium (Cr), manganese (Mn), silicon (Si) or vanadium (V) while, simultaneously, carburizing the main steel component, i.e., iron (Fe). The same oxidizing-carburizing effect takes place in other atmospheres such as dissociated alcohol atmospheres, e.g. N2-methanol and N2-ethanol.
The oxidizing-carburizing effect is undesired. In many instances, oxides located at the grain boundaries of metal weaken the surface and accelerate fatigue cracking or corrosion in the subsequent service. It is well recognized that countermeasures are costly, time, energy, and capital equipment intensive, and/or not available when carburizing thin wall steel components or net-shape surfaces. For carburizing treatments, these countermeasures may involve extending the carburizing cycle time in the furnace in order to develop an excessively thick carbon-rich layer in the metal surface and mechanical removal of the most external, oxide-affected portion of this layer in the following machining operations. In other treatments, the oxidizing-carburizing effect may deteriorate the surface appearance of annealed metal by forming spots of oxide films. Moreover, the oxidizing potential of these atmospheres may inhibit or completely prevent carburizing and the related, diffusional surface treatments of highly alloyed steels such as stainless steels and various types of tool steels and superalloys.
To avoid this oxidizing-carburizing effect, the metals industry may use oxygen-free atmospheres which, at the gas inlet to processing furnace, can contain technically pure N2, H2, NH3, HC, and their combinations and mixtures, with optional argon or helium additions, but not air, CO, CO2, H2O or alcohols and their vapors. It is well known that elimination of oxygen (O2) containing gases from the furnace atmosphere, including air, CO, CO2, H2O, or alcohols and their vapors, is an effective solution to the problems outlined. This can be realized by using HC, HC—N2, or HC—H2 gas stream during low-pressure carburizing treatments in vacuum furnaces, where all air and moisture have been pumped out from the furnace volume in the preceding operations. The O2-free, N2—HC and N2—H2—HC atmosphere treatments have also been used with various degrees of success in the atmospheric (e.g., ambient, 1-atm pressure) furnaces. Here, the main complicating factor is a difficulty in excluding leakage of ambient air into the furnace. Although very popular and relatively inexpensive, the 1-atm-pressure furnaces cannot offer the level of atmosphere control found in vacuum furnaces. Additional factors encountered may include release of moisture from the ceramic refractory of the furnace and minor leaks of combustion flame from radiant heating tubes to the treatment space of furnace.
Carburizing process control in the conventional, endothermic and dissociated alcohol atmospheres containing oxygen is based on the equilibrium of the carburizing-decarburizing reaction on the surface of iron. The reducing potential of the atmosphere, associated with its carburizing potential can be measured with zirconia probes, frequently called oxygen or carbon probes. This process control method cannot be used with the O2-free atmospheres described above because there is no equilibrium; the metal is carburized proportionally to the exposure time, temperature, and the flux or transfer of carbon-bearing species from the atmosphere to the surface. Here, the ultimate carburizing limit under the ordinary heat treatment conditions is the conversion of the substantial or entire metal volume into carbide by the HC-component of the atmosphere, which is an undesired outcome.
The most popular method of solving the process control challenge in vacuum furnaces involves a trial-and-error based development of carburizing recipes that regulate the mass flux of HC gas. The key variables involve the type of HC gas used, its flowrate, temperature, pressure, carbon boosting and diffusing time required for producing desired carbon concentration profile under the surface of the metal part, composition and total surface area of the parts treated. Since these variables can be precisely controlled, the number of trials needed to develop a particular recipe is small. Based on those recipes, the subsequent production runs can be automated and supported with popular computer-calculated diffusion models predicting in real-time the development of carbon concentration profile in metal.
The process control challenge is more difficult in the case of 1-atm-pressure furnaces which, as mentioned above, are less precise than vacuum furnaces and involve a number of additional, sometimes uncontrollable processing variables such as air and combustible gas leakage or moisture desorption. The development of recipes may require more trials than in the case of vacuum furnaces, and the carburizing cycle including carbon boost and diffuse may necessitate real-time, dynamic corrections to the processing parameters using some type of a feedback loop.
Various carbon flux probes, microbalance instruments and schemes have been developed over the years to address the challenges of process control in non-equilibrium as well as equilibrium atmospheres. Illustrative examples include those disclosed in the following references: U.S. Pat. Nos. 4,035,203; 4,591,132; 5,064,620; 5,139.584; and 7,068,054; EP Pat. No. 0353517A2; and U.S. Publ. No. 2008/0149225A1. U.S. Pat. No. 7,068,054 describes a sensor probe, measurement system and measurement method for directly measuring solute concentration profiles in conductive material components at elevated processing temperatures. US Publ. No. 2008/0149225 provides a method of treating a metal part in an atmospheric pressure furnace using an oxygen free controlled gas. Their applicability to non-equilibrium atmosphere carburizing in 1-atm-pressure furnaces is, nevertheless, limited, as well as the reliability and lifetime of carbon-flux probes in industrial, non-stop production environments.
Accordingly, there is a need in the art to provide a method and/or apparatus to enable or improve the development of process recipe and the subsequent dynamic control when no suitable carbon flux probe is available and/or when the continuous use of the probe in the non-equilibrium atmosphere used poses reliability problems.